Split the vanilla pod length ways and scrape out the seeds and add them and the pod and the milk to a pan. Scald the milk ie bring almost to boiling point and then remove from heat. Whisk the egg yolks and sugar until thick and white. Pour the milk into the egg mix and stir well. Return the mix back to the pan and cook on a low heat while stirring continuously for about 10-15 minutes. Once the mix is thick enough to coat the back of a wooden spoon remove from the pan from the heat and place in a bowl of cold water whilst continuing to stir the mix until cool. (Optional) Add the vodka (or other spirit) to give the finished ice cream a softer texture. (Optional) Once cool poor the custard into a 1lr container and allow to thicken in the fridge for 4 hours, this allow the flavour to develop and for the fat droplets to crystallise which is important for the final texture. Place the cool mix into the freezer.